1. Field of the Invention
The invention relates to a process for the biological purification of polluted water, such as waste water, wherein the polluted water is successively subjected to an anaerobic, an anoxic and an aerobic treatment in the presence of micro-organisms in order to reduce the nitrogen and phosphorus contents of the water.
2. The Prior Art
The ever increasing eutrophication of rivers, lakes and seas has resulted in more and more countries making requirements for the purification of waste water, in particular for the removal of nitrogen and phosphorus, and in the existing requirements becoming more and more strict.
Various processes for the biological purification of waste water are known wherein at least a partial removal of nitrogen and phosphorus from the waste water is effected. Such known processes for the purification of waste water using bacteria are disclosed in i.a. DK patents Nos. 149,767 and 153,832.
In the known processes for the biological removal of nitrogen and phosphorus from waste water, the amount of easily decomposable organic matter contained in the waste water is often a limiting factor in the effectiveness of the processes, and where the waste water content of easily decomposable organic matter is insufficient, it is often necessary to add organic matter in order to obtain a desired degree of purification.
The biological removal of phosphorus from waste water is a result of the presence of phosphorus-accumulating bacteria which, in anaerobic conditions, absorb easily decomposable organic matter from the untreated waste water, which organic matter is stored in the form of, e.g., polyhydroxy butyrate (PHB). The bacteria acquire the energy to perform such storing by decomposing polyphosphate from an intracellular storage. Hereby orthophosphate ions are produced which are released to the liquid phase.
When the phosphorus-accumulating bacteria are subsequently subjected to aerobic conditions, the storage of organic matter is consumed as oxygen works as an oxidant. The bacteria use the energy thus produced partly for the absorption of orthophosphate ions from the liquid phase and for the accumulation thereof in the form of polyphosphate and partly for the basic metabolism and growth of the phosphorus-accumulating bacteria.
The removal of phosphorus from the waste water is subsequently achieved by removing the excess production of phosphorus-accumulating bacteria at a point in the process when their polyphosphate storages are full.
The removal of nitrogen is based on a nitrification followed by a denitrification. The nitrification which is effected in aerobic conditions consists in oxidiation of ammonia nitrogen into nitrate simultaneously with a decomposition of available organic matter, if any, whereas the denitrification which is effected in anoxic conditions consists in the consumption of organic matter using nitrate ions as oxidants. In this oxidation nitrate nitrogen is reduced to free nitrogen (N.sub.2) which is released in its gaseous form.
Part of the phosphorus-accumulating bacteria are also capable of consuming stored organic matter in anoxic conditions as nitrate acts as an oxidant. As it is the case when the consumption of organic matter is effected with the use of oxygen as an oxidant, the energy produced in anoxic conditions is used partly for the absorption and for the accumulation of phosphate in the form of polyphosphate and partly for the growth of the phosphorus-accumulating bacteria.
When nitrate acts as an oxidant as it is the case in the anoxic conditions described above, a reduction of nitrate into free nitrogen takes place as disclosed above, the free nitrogen being released in gaseous form. This means that the organic matter absorbed by the phosphorus-accumulating bacteria using nitrate as an oxidant is used for phosphorus absorption and accumulation as well as for denitrification.
Tests have shown that in an active-sludge method, such as the ones described above wherein a mixture of micro-organisms are successively subjected to anaerobic, anoxic and aerobic conditions, only about half of the phosphorus-accumulating bacteria are capable of absorbing and accumulating phosphate ions in anoxic conditions and thus of using nitrate as an oxidant. Thus, only half of the total amount of organic matter absorbed by the phosphorus-accumulating bacteria is used for denitrification.
If all of the phosphorus-accumulating bacteria were capable of using nitrate as an oxidant, it would be possible to obtain an improved nitrogen removal without the addition of organic matter to waste water which is comparatively poor in organic matter as, in that case, there would be more organic matter available for the nitrogen removal, i.e., the denitrification.